This invention relates to a method and apparatus for removing scale consisting essentially of Fe.sub.2 O.sub.3 and deposited on the surfaces of fuel rods which are arranged in grid form to constitute a fuel rod assembly. In the art of a boiling water type nuclear reactor, such scale is usually termed a "clud".
As is well known in the art, in a boiling water type nuclear reactor, a reactor core comprising a plurality of fuel rod assemblies and equipped with control rods is disposed in a pressure vessel, and water acting as a coolant and decelerating medium is circulated through the core to generate steam.
As shown in FIG. 1, a typical fuel rod assembly 6 comprises tubular sheaths 20 with their upper and lower ends sealed by upper end plugs 18 and lower end plugs 19 respectively, and pellet shaped fissionable fuels 21 are contained in each sheath 20. Each lower end plug 19 is tapered to be supported in vertical alignment by a supporting opening 22 provided for a lower tie plate 14 while each upper end plug 18 has an extension 23 received in a supporting opening 24 extending through an upper tie plate 15.
Some of the supporting openings 22 of the lower tie plate 14 are provided with screw threads for receiving fuel rods 16 provided with threaded lower end plugs 19. An extension 23 of the upper end plug 18 of each fuel rod 16 extends upwardly through the supporting opening 24 of the upper tie plate 15, and a holding nut is threaded onto the upper end of each extension 23 so as to assemble the upper and lower tie plates 14 and 15 and the fuel rods 16 into an integral fuel assembly 6. The spacings between fuel rods 16 are maintained by a plurality of spacers 17.
The fuel assembly 6 is housed in a channel 26, shown in FIG. 1, having a square cross-section. A clip 27 having a perforation is welded to the upper end of the channel 26 so as to connect the channel 26 to the tie plate 15 with a bolt extending through the clip 27. When the nut is removed from the bolt, the channel 26 can be readily mounted and dismounted while sliding along the upper and lower tie plates 14 and 15 and spacers 17.
During the operation of a nuclear reactor, a large quantity of cooling water is circulated through the spaces between respective fuel rods.
Although the cooling water is used for the purpose of decelerating neutrons and transmitting the heat generated, it is circulated together with a corrosive product (scale) generated by the internal structure of the nuclear reactor. Since the fuel rods are at the highest temperature among various internal structures they adsorb most of the scale formed. Consequently, scale will accumulate with time on the surfaces of the fuel rods to decrease the rate of heat transfer thus causing the surface temperature to rise.
When transferring and treating a fuel assembly in which a large quantity of scale has been deposited on the surfaces of the fuel rods to a working factory, the transfer and treatment of the rods should be made very carefully because the deposited scale is highly radioactive.
Although the cooling water is highly purified before use, it still contains a certain amount of impurities that form scale.
One approach to this problem is to strictly control the quality of cooling water so as not to convey the scale into the reactor core, but since a large quantity of scale is generated by the internal structure it is impossible to prevent deposition of the scale on the surfaces of the fuel rods.
Accordingly, the only remaining measure is to periodically remove the scale, but since the scale is highly radioactive it is necessary to carefully and safely remove the scale and to safely discard the removed scale.